1. Field of the Invention
This invention relates to the use of charge transfer complexes and more particularly, to functional devices using such complexes in which variations of electric, optical or electromagnetic characteristics in the device are utilized when external energy such as electric, optical or electromagnetic energy, pressure and/or temperature is applied to the device. The functional device can be utilized by itself or in combination with other devices as switching devices, electric and optical memories, optical devices utilizing optical waveguide, a photovoltaic effect and photoconductivity, display devices and sensors.
2. Description of the Prior Art
As is well known in the art, organic charge transfer complexes have wide utility in various fields in which their specific characteristics including electric conductivity or semiconductivity, electromotive force, dielectric properties, photoconductivity or the like are utilized. For instance, the complexes are utilized as polarized or ionized. Alternatively, a neutral to ionic phase change of these complexes are used to make various electric, electronic, optical or electromagnetic devices.
It is known that a certain type of organic molecule crystal undergoes a phase transition of from a neutral crystal which may be a van der Waals crystal to an ionized crystal wherein most constituent molecules are substantially ionized. For instance, it has been found that a single crystal of a charge transfer complex, for example, of tetrathiafulvalene (TTF) and chloranyl (CA) undergoes the phase transition at a relatively low pressure of about 10 Kbar. In addition, when the temperature is lowered down to not higher than 80.degree. K. even at a normal pressure, this single crystal undergoes the neutral-ionic phase transition. The neutral-ionic phase transition is considered to result from the balance between an energy loss of I.sub.D -E.sub.A wherein I.sub.D is an ionization potential and E.sub.A is an electron affinity and a gain of the Madelung energy, aV wherein "a" is a Madelung constant and V is a coulomb energy of a D.sup.+ A.sup.- pair wherein D is an electron donor and A is an electron acceptor in case where the electron donor and the electron acceptor are converted from a neutral state (D.degree.A.degree.) into an ionized state (D.sup.+ A.sup.-). If I.sub.D -E.sub.A &gt;aV, the complex is neutral and if I.sub.D -E.sub.A &lt;aV, the complex is ionized. In this connection, however, it has been recently found that the neutral to ionic phase transition does not occur only by a simple change in the Madelung energy, but is greatly influenced by the charge transfer interaction between the donor and acceptor molecules and also by the electron-lattice interaction. For instance, the neutral-ionic phase transition of crystals of tetrathiafulvalene and chloranyl caused by application of temperature or pressure is accompanied by the dimerization lattice strain. This means that aside from the energy balance depending upon the coulomb force, the electron-lattice interaction which will cause Peierls deformation is important.
On the other hand, a Cu and tetracyanoquinodimethane (which may be hereinafter referred to simply as TCNQ) complex is considered to undergo the neutral-ionic phase transition at room temperature under normal pressures. This complex may be formed by dissolving TCNQ in acetonitrile and subjecting the solution to reaction with Cu. When the complex is sandwiched, as a layer, between Cu and Al electrodes and is applied with a voltage, a switching phenomenon is observed where the resistance is kept high by application of a voltage up to a certain voltage, and becomes low when the applied voltage exceeds the level. Presumably, this is because (Cu.sup.+ TCNQ.sup.-).sub.n is converted into a neutral phase where Cu.sub.x .degree. and (TCNQ.degree.).sub.x are formed. This ionic to neutral phase change may be caused by application of light.
With the organic molecule crystals such as, for example, of tetrathiafulvalene and chloranyl, however, low temperatures or application of high voltages is necessary to cause the phase transition to proceed.
On the other hand, crystals of (Cu.sup.+ TCNQ.sup.-).sub.n which undergo the phase transition at normal temperatures and pressures are relatively unstable and are not reliable with respect to reproducibility. In addition, it has been reported that the reason why the electric conductivity increases after the transition to the neutral phase by the switching phenomenon is due to an increase in amount of the Cu in the charge transfer complex because of the evaporation of the TCNQ molecules caused by the Joule heat. Accordingly, the reproducibility of the switching phenomenon is not necessarily reliable, thus presenting a practical problem in application of the complex as a function device.
On the other hand, charge transfer complexes consisting of electron donors and acceptors are, in most cases, applied using their inherent electric, optical and physical properties without use of such a specific phase transition as described above. For instance, many reports have been made with respect to conductivity, electromotive force, dielectric properties and photoconductivity of charge transfer complexes. However, these complexes are usually polarized isotropically. When the direction of polarization is isotropic, any electric charge induced by the polarization do not produce, or the photovoltaic force is offset and lessened. In addition, dielectric characteristics are also offset.